Highest stresses seem to form at the surfaces of the parts especially at the edges and at the bottom where part is connected to the build platform. These stresses exceed the yield strength of the MS1 causing the distortions. Distortion orientation trend can be seen to be inwards.
This is most likely caused by contraction happening in melt pool when it cools down and
solidifies causing the whole part to shrink. Highest distortion can be seen in part 1. This distortion is about 1 mm which is a lot higher than distortion in part 2 even though part 2 has higher stresses. In part 2 the arch is supported from both sides. Part 1 overhang is not supported which allows it to distort freely. Using supports could reduce distortions. Stresses in part 3 were distributed more evenly. Distortions were concentrated in corners which lacks support from structure itself. It seems thin horizontal geometries has tendency to distort.
Same distortions cannot be seen on inclined plates. Distortions could be avoided with orientation of the part.
6 CONCLUSIONS
Aim of this thesis is to characterize thermal effects of L-PBF on manufactured part, based on simulation. Characterization of deformations by simulation could save resources in test prints by optimizing AM process before first process. Thesis was conducted with literature review of simulation process and geometries. Experimental part was done with simulation on 3DExperience using thermo-mechanical method.
This study was carried out as part of Metal 3D Innovations (Me3DI) project funded by European Regional Development Fund (ERDF). Aim of this project is to form knowhow cluster for metal 3D printing to South Karelia. Project started 1.9.2018 and it ends 31.12.2020. Thesis was produced in Research Group of Laser Material Processing and Additive Manufacturing (3D Printing) of Department of Mechanical Engineering at LUT University
Simulated geometries were decided to be similar to test parts found in literature review. This was done to prevent the thesis from being too broad and to verify simulation results.
3DExperience platform was used to model and simulate the parts. Thermo-mechanical method was used for simulation. Used parameters were EOS M290 AM devices parameters for tool steel MS1.
Simulating AM process could help in manufacturing of multiple parts in the same process.
This could save money and time but requires further studies
Material is melted in L-PBF with high energy laser. This temperature increase causes temperature expansion and contraction when it cools down. This thermal behavior creates distortions and residual stresses in the part. Prevention of this issue requires further studies.
In literature review, it was found that arch structures, overhangs and thin horizontal plates were problematic in AM. Archs and overhangs could be manufactured without support structures but it would cause heat buildup and allow increased freedom to distort.
Residual stresses formed mostly on the surface of the parts and at the interface of the part and build platform. Highest stresses were concentrated on corners. These stresses would often exceed yield strength of the material causing further distortions. Distortions formed on less supported parts. Highest distortions were encountered in thin unsupported horizontal structures. These distortions were smaller if structure was supported. On inclined structures these stresses were not seen. Stresses could be avoided with orientation of the part.
Printing overhangs without supports reduces print time and amount of material used. These reductions come at a cost. Supports also helps conducting heat better than raw powder. Better heat conduction decreases heat buildups. These heat buildups caused dross to form at the surface.
7 FURTHER STUDIES
It is possible to simulate build platform and support removal on 3DExperience. This would give more accurate results since removing them causes distortions and stresses to shift.
Simulating heat treatment could give results on how much of the stresses and distortions are recoverable. Simulation used in thesis did not get results for surface quality. Further studies could be conducted to achieve this.
3DExperience has possibilities to gain other results than residual stress and distortions from simulation. Further studies of possible results could be useful.
Multiple methods exist to simulate 3D-printing process. In further studies these methods could be used and compared to each other to study the differences in results. Real print could also be made to study how close the simulation results are.
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APPENDIX I, 1 Full list of parameters used for simulation
Property Value
Build platform mesh size [mm] 3
Range 1
start 1
Pending time before recoating [s] 0
Recoating time [s] 0
Recoating break time [s] 9
Pending time after recoating [s] 0
Start temperature [K] 300
Gas inlet direction [°] 0
Gas flow [𝑚
3
𝑠 ] 0
Scanning iteration of first slice 1
Part order scanning default
Distance between part and build platform false
Minimum distance between parts [mm] 1
Clearance on build platform border [mm] 1
Minimum height between parts [mm] 1
Nesting options Nesting 2D with bounding box of the part
Zone type Zones as surfaces
Shape type Wired support
Pattern grid
With envelope false
Spacing [mm] 3
Direction angle [°] 0